The broad objective of this research project is to obtain some further insight into metabolic processes and factors regulating their activity. A study of the more primitive marine forms has been initiated in the hope that it will furnish clues as to basic processes which may have been elaborated upon by vertebrates during evolutionary development. Attention has been focused primarily on the horse shoe crab, Limulus polyphemus. Studies to date on this animal include the availability of free fatty acids for energy purposes; the identification of red pigments in the coxal gland, a primitive kidney, as carotenoids; and the isolation and identification of biliverdin as one of several egg pigments.